The art of printing by offset lithography involves the use of thin, flexible plates which are basically planographic, that is without relief, and which are treated so that the surfaces have areas which attract or repel oil and water to create areas of image differentiation. To mount the litho plates on the surface of the press plate rolls, it is essential to insure that the plate conforms closely to the outer surface of the plate roll. Mounting is usually effected by forming a longitudinal slot in the plate roll and inserting thereinto one of a variety of biasing or clamping devices called lockups. Often the leading edge of the plate is bent back on itself so that the bent portion can fit over the edge formed in the roll by the longitudinal slot and then be held in position by some form of spring or clamp. Alternatively, mechanisms have been provided in which the plate leading edge has been rigidly clamped.
An example of the type of plate lock-up mechanism in which the leading edge of the plate is clamped can be seen by referring to U.S. Pat. No. 4,421.024. In this arrangement, the plate leading end portion E is held between two parts 5 and 6 which are of an approximate trapezoidal shape. The two parts 5 and 6 are urged inwardly to clamp the end of the plate within slot 3 by means of the spring 8. The trailing end of the plate, which is identified by the letter A, is held between the sloped surface of part 6 and the similarly sloped wall 10 of slot 3.
Another example of a plate lock-up is seen in U.S. Pat. No. 3,757,691 where the leading edge of plate 11 is held by one leaf of a spring member 16 while the trailing edge of plate 11 is held by another finger of the spring member 16. An elongated cam 18 is used to change the spring pressure exerted against the plate for purposes of unlocking and locking the plate in and from operating position, respectively.
A plate lock-up mechanism which is somewhat related to that of U.S. Pat. No. 3,757,691 is U.S. Pat. No. 3,626,848 where a spring member is used to hold down the leading edge of plate 11 around the undercut side of the slot and cam 40 is used to urge the hook 35 against the trailing end 13. Examples of other plate lock up mechanisms may be found in U.S. Pat. Nos. 3,608,847 and 3,095,811.